1. Field of the Invention
The present invention relates to a DC/DC converter for converting the power supply voltage to be input into a predetermined DC voltage, and more particularly to a DC/DC converter having a soft start function.
2. Description of the Related Art
A method of a DC/DC converter is installing a switching element between a terminal for inputting power supply and a terminal which is connected to a load and outputs a predetermined DC voltage, and maintaining the predetermined DC voltage by switching this switching element. This method has been widely used because high efficiency can be achieved with a compact size (e.g. Japanese Patent Application Laid-Open No. H9-121535). And this method allows to construct various DC/DC converters, such as the step-up type, step-down type and step-up/down type depending on the circuit configuration around the switching element. In these DC/DC converters, a soft start circuit is normally installed to control rush current, which flows into the switching element when power is started up.
FIG. 5 is a circuit diagram of a conventional DC/DC converter. This DC/DC converter 101 is a step-up type, and is comprised of an NMOS transistor 114 which is a switching element, and a coil 115 for storing boosting energy between the drain thereof and power supply (Vcc). Between these connecting points and the load side output terminal (OUT), a reverse blocking diode 116 and a smoothing capacitor 117 are connected. In this circuit configuration, the voltage of the load side output terminal (OUT) is adjusted by controlling the ON time of the NMOS transistor 114 using the feedback circuit, which will be described next.
The feedback circuit is comprised of an output voltage detection circuit 106, error amplifier 111, triangular wave generator (TRI) 112, comparator 110 that has two non-inversion input terminals, and output buffer 113. In other words, the output voltage detection circuit 106 detects the voltage of the load side output terminal (OUT), the error amplifier 111 receives the output voltage thereof, amplifies it and outputs it to one of the non-inversion input terminals, the comparator 110 compares this output voltage and the triangular wave voltage generated by the triangular wave generator (TRI) 112, and this comparison result output is input to the gate of the NMOS transistor 114 via the output buffer 113, so as to control the ON time of the NMOS transistor 114.
The soft start circuit 105 is also installed to control the rush current which flows into the NMOS transistor 114 when power supply (Vcc) is started up. This circuit is comprised of a constant current source 122 and a capacitor 123, and generates a voltage which rises gradually. This voltage is output to the other non-inversion input terminal of the comparator 110. And when the power supply (Vcc) is started up, the output voltage of the soft start circuit 105 and the above mentioned triangular wave voltage are compared by the comparator 110.